Method of extending the life of electrolytic polishing baths



Patented May 9, 1944 METHOD OF EXTENDING THE LIFE OF ELECTROLYTIC POLISHING BATES Henry A. Holden Pray, Columbus, Ohio, assignor to Battelle Memorial Institute, Columbus, Ohio, a corporation of Ohio No Drawing. Application November 4, 1939, Serial No. 302,852

6 Claims. (Cl. 204-440) This invention relates to a method of extending the useful life of electrolytic polishing baths. More particularly, the invention pertains to a method of regenerating spent electrolytic polishing baths comprising as active ingredients mixed sulfuric and phosphoric acids that have become saturated with nickel salts during the operation of said baths.

Reference is made to my application entitled: "Method ofextending the life of electrolytic polishing baths, SerialNo. 241,948, filed November 23, .1938.

In the polishing of metals by electrolytic processes it is customary to make the metal article to be polished the anode in an electrolytic solution of suitable composition and to pass therethrough a current of sufllcient anodic current density for a length of time sufficient to produce the desired degree of polish on the metal. Suitable compositions for polishing baths usually include sulfuric acid as one of the'active ingredients, to

which phosphoric acid is advantageously added. Hydrofiuoric acid, chromic acid, or like acids, may be substituted for phosphoric acid, or, if desired, organic materials such as glycerol, glycyrrhiza, and the water soluble glycols, may be added.

After prolonged operation, an electrolytic polishing bath becomes exhausted. i. e., the bath can no longer be effectively used to polish metals. The eflicient life of a bath 'varies greatly, depending upon its composition and on the particular metal or alloy that has been polished therein.

containing mixed sulfuric and phosphoric acids as principal active ingredients. When such a bath is used for the polishing of nickel and its alloys, nickel salts and other products of electrolysis accumulate in the bath during the operation of the same. Eventually, when a certain concentration of nickel salts has been reached,- the bath no longer operates effectively. According to the present invention, thus exhausted baths may be regenerated by a heat treatment precipitating such accumulated nickel salts as filtration or other means. added in an amount suflicient to replace the i 1 nickel sulfate in a form suitable for removal by Sulfuric acid may be quantity of sulfate removed as nickel sulfate, to build up the sulfuric acid concentration of the bath to optimum operating strength.

It is therefore an important object of the present invention to provide a method for extending the useful life of electrolytic polishing baths by reducing the concentration of accumulated products of the electrolytic action and by restoring to optimum operating strength'the active or principal ingredients of the electrolytic polishin baths. 7

It is another important object of this invention to provide a simple and efllcient method for the restoration of exhausted nickel polishing baths comprising sulfuric and phosphoric acids as active ingredients.

It is a further important object of this inven-- tion to provide a method of heat treatingelectrolytic polishing baths comprising as active in-. gredients sulfuric and phosphoric acids containing nickel salts accumulated during the operation of said baths, to precipitate therefrom said accumulated nickel salts as nickel sulfate in a form suitable for removal by filtration, and to restore said active ingredients to operating efii ciency.

Other and further important objects of this invention will become apparent from the following description and appended claims.

I have found that in the electrolytic polishing I of nickel or nickel alloys by means of electrolytic baths of the following composition:

Broad I Narrow Preferred range I range range I I l 1 Per com I Per cent Per cent Sulluricacid.. 5to85 15mm l5t020 Orthopbosphoric acid 5 to 85 15 to 63 to 67 Water I Not ove 50 Not over 50 Balance satisfactory polishing results can be obtained so and HaPO4. The use of percentages by weight rather than byv volume and the designation of the respective acids as 100% acids, rather than as commercial strengths, is in accordance with the disclosurein the co-pending abandoned application Serial No. 241,948, filled November 23, 1938.

I have further found that at the usual operating temperature of the hereinabove specified polishing baths, i. e., at about 120 F., precipitation commences in said baths when the nickel content reaches about 2.5 per cent. The precipitation formed is extremely fine-grained, settles very slowly, and can be filtered oif only with great difiiculty. The compositionof this precipitate can not easily be determined analytically, for the precipitate adsorbs free acids from the baths. However, such analytical data as I have bee able to obtain indicate said precipitate to be an acid nickel phosphate.

' I have also noted that there is a decided increase in the viscosity of said baths on prolonged use which is indicative of yet other changes occuring therein. Possibiythere is a transformationof orthophosphoric acid to a poly acid. That these changes, whatever their nature may be, are important factors afiecting the operating efliciency of said baths is shown by the fact that reducing the nickel content of exhausted polishing baths to about 2.5 per cent by filtering ofi nickel salts precipitated therein does not restore the operating efliciency. In other words, the loss of operating efliciency of electrolytic polishing baths .on prolonged operation is not due simply to an accumulation of anodically dissolved nickel therein, and is not curable merely by removal of such anodically introduced nickel. The active acid ingredients of the polishing baths are also transformed chemically on prolonged operation into nonefilcient forms, which transformation is per se sufiicient to render the baths inoperative. The exhaustion of an electrolytic nickel polishing bath is therefore a complex process affecting the 4 bath as a whole.

to lower hydratesoi' nickel sulfate, depend on the intensity and time of heat treatment. A heat treatment equivalent to the above disclosed treatment at about 180 F. for from one to three hours will reduce the amoimt of dissolved nickel in solution to about 0.5 to 1.5 per cent, the undissolved nickel being removable by filtration.

Mere heating to 180 F. followed by cooling yields a more easily handled and more rapidly settlingprecipitate of nickel sulfate, but the re-' sulting sludge has better filtering properties and the amount of nickel retained in the bath is less if the heat treatment at about 180 1". is prolonged for from one to three hours. Slow cooling promotes crystal growth.

Heating, preferably at about 180 F. for from 1 to 3 hours, as disclosed hereinabove also mate-' rially reduces the viscosity of the baths, indicating'a corresponding transformation of the acid exact temperature used is not critical. Merely dependent upon increasing the salt concentra- Preferably the boiling tion of the bath by evaporation of the solvent.

In fact, I prefer not to have any losses by evaporation during the heat treatment step.

Such heat treatments efl'ect the precipitation of nickel salts, suspended and dissolved in said baths, as a lower hydrate of nickel sulfate, prob-' ably as monohydrated nickel sulfate,.or as a mixture of lower hydrates of nickel sulfate. These lower hydrates are salts which are less soluble in said baths than the nickel salts .originally formed by anodic dissolution and are precipitated as large, rapidly settling crystals that can easily be filtered off. The crystal size and rate of settling, as well as completeness of conversion ingredients of said baths. Possibly there is a reversion of a polyphosphoric acid to orthophosphoric acid.

The precipitated nickel sulfate is suitably removed by filtration. First allowing the precipicycle may be repeated indefinitely. I have,

for instance, successfully operated a polishing bath through a sufiicient number of such cycles to remove, as nickel sulfate, an amount of anodically dissolved nickel equal to 43 per cent, by weight, of said bath. At the conclusion of said prolonged cyclic operation the bath showed the same efiiciency as at the commencement thereof.

Broadly speaking, the principles of the present invention comprise a regenerative heat treatment of exhausted electrolytic polishing baths containing sulfuric and phosphoric acids as active ingredients, to precipitate anodically dissolved nickel salts as a less hydrated form of nickel sulfate andto restore the activity of the acid ingredients of said baths. This heat treatment is effective within a wide range. The heat treatment can also be eiiectively used in baths containing as active ingredients sulfuric acid either alone or in combination with acids other than phosphoric acid, precipitation of anodically dissolved nickel being efiected while reversion of non-active forms of acid ingredients to active forms takes place.

Although it is believed that the nickel sulfate precipitated as a result of the above described heat treatment is largely in the form of the mcnohydrate, the possibility of other forms being present containing less than 6 molecules of water of hydration is not excluded.- By the term "lower hydrates of nickel sulfate," I mean, therefore, to include the monohydrate itself and mixture of hydrated nickel sulfates containing less and I, therefore, do not propose limiting the I claim as myinvention: 1. The method of restoring 'lost operating efllciency of a used electrolytic polishing bath which contained originally from to per cent sulfuric acid and from'63 to 67 per cent phosphoric acid, by weig'ht,-j. the balance being largely water,

and which also contains nickel salts" dissolved anodically during use, which method comprises heating said bath to a temperature between about lac; F. anti'the boiling point of said bath, allow ing said bath to cool, and removing precipitated I nickel sulfate therefrom. a

2. The method of restoring lost operating efiiciency of a used electrolytic polishing bath which contained originally from,15 to 20 per cent sulfuric acid and from 63 to 6'7 per cent phosphoric acid, by weight, thebalance being largely water, and whichfalso contains nickel salts dissolved anodically during use, which method comprises heating said bath to about 180 R, allowing said, bathto cool removing precipitated nickel sulfate 180 F and the boiling point of said bath for a jirriqiiot exceeding three hours, allowing said cool, removingprecipitated nickel sulfate as a lower hydrate of nickel sulfate and to retherefrom and adding acid ingredients to replace losses-thereof. 7

4. The method of extending the useful life of a used electrolytic polishing bath consisting essentially of phosphoric and sulfuric acids and water, in which sulfuric acid is an active ingredient and in which anodically dissolved nickel salts are present, which method comprises heating said bath'to a sufilciently high temperature between about F. and the boiling point thereof and for a sufficient period of time to effect precipitation of said nickel salts as a lower hydrate of nickel sulfate and to reactivate said bath.

5. The method of extending the useful life of a used electrolytic polishing bath consisting'essentially of phosphoric and sulfuric acids and water, in which sulfuric acid is an active ingredient and in which anodically dissolved nickel salts are present, which method comprises heating said bath to betweenabout 180 F. and the boiling point thereof without substantial evaporation. to effect precipitation of said nickel salts activate said bath, cooling and removing said nickel sulfate.

6. The method of restoring lost operating efficiency of a used aqueous electrolytic polishing bath containing sulfuric and phosphoric acids as active ingredients in which dissolved nickel salts are present, which method comprises heating said bath to at least about 180 F. to reactivate the bath and to convert a portion of the nickel salts present into and precipitate them as a lower hydrate of nickel sulfate, removing the thus precipitated nickel sulfate, and adding sulfuric acid to said bath to replace losses of said acid.

HENRY A. HOLDEN PRAY. 

